1. Field of the Invention
The present invention relates to an image forming apparatus provided with a load and a power source for driving said load, and a safety switch for switching an electrical connection and a disconnection between said load and said power source in conjunction with the opening and closing of a cover provided on the image forming apparatus so as to be operational.
2. Description of the Prior Art
Image forming apparatus such as digital copying machines, laser printers and the like typically provide a deflection device such as a polygonal mirror, galvano-mirror or the like to modify a laser beam modulated in accordance with image information.
The aforesaid deflection devices are typically driven at extremely high speed to accelerate image recording. A polygonal mirror, in particular, is rotatably driven at a high speed of 5,000 rpm via a polygonal mirror motor (hereinafter referred to as "polygonal motor"). In the description that follows, examples pertain to a laser printer provided with the aforesaid polygonal motor.
Laser printers are typically provided with an operational cover to allow easy processing of paper jams, routine maintenance, inspections and the like. In consideration of safety factors during paper jam processing and maintenance inspections, a safety switch is provided for switching an electrical connection and a disconnection between the load and power source for driving said load in conjunction with the opening and closing of the aforesaid cover.
FIG. 1 shows an example of a drive circuit for a laser printer. As shown in the drawing, the aforesaid drive circuit comprises a polygonal motor drive circuit 101 for driving a polygonal motor 110, a laser beam drive circuit 102 for driving a semiconductor laser that generates a laser beam, a main motor drive circuit 103 for driving a main motor 111 that drives the paper feed device and fixing device and the like, and other loads 104 which are connected to a low-voltage power source 100 via the safety switch SW. In the previously mentioned state wherein the cover is closed, the safety switch SW is switched ON, and the aforesaid polygonal motor drive circuit 101, laser beam drive circuit 102, main motor drive circuit 103 (hereinafter referred to as "drive circuits 101 through 103"), and load 104 are electrically connected to the low-voltage power source 100. When the aforesaid cover of the laser printer is opened for routine maintenance inspection, the safety switch SW is turned OFF in conjunction with the operation of opening said cover, such that the aforesaid drive circuits 101 through 103, and load 104 are electrically disconnected from the low-voltage power source 100.
Disadvantages inherent to the above-described laser printer are described below.
Since the polygonal mirror is rotated at exceptionally high speed during image recording, when the cover is opened while the polygonal mirror is rotating, said polygonal mirror continues to rotate even after the safety switch SW is turned OFF due to the force of inertia. Therefore, although the aforesaid drive circuits 101 through 103, and load 104 are electrically disconnected from the low-voltage power source when the safety switch SW is turned OFF, the polygonal motor 110 continues to generate a regenerative electric force flowing in the arrow direction in the drawing, relative to the main motor drive circuit 103 and load 104. When this regenerative electric force is generated, a disadvantage arises inasmuch as a voltage is applied to the laser drive circuit 102, main motor drive circuit 103 and load 104 connected to the polygonal motor drive circuit 101, such that standard voltage safety levels are not satisfied. This disadvantage is such that the safety switch SW does not fulfil its function.